Developers of ambulatory care information systems are challenged by the difficulty of meeting the simultaneous and often conflicting, goals of 1) capturing and electronically representing clinical data with sufficient expressibility to provide adequate and accurate documentation of the patient encounter, and 2) utilizing standardized terminologies, such as diagnostic and procedure codes, which are inadequate to express the details of the patient encounter, but which facilitate communication with other types of computer based systems. The aim of this study is to compare selected standardized coding and classification systems for their ability to represent the terms used by physicians (MDs) and nurse practitioners (NPs) to document in the ambulatory care record. Each system will be tested with a sample of 1000 (500 MD and 500 NP) patient encounters for human immunodeficiency virus (HIV) care. The unit of analysis for this study is the clinical term. Two types of terms will be evaluated using a semi-automated lexical matching approach: 1) the natural language terms charted by the MDs and NPs, and 2) the base concepts of the natural language terms which will be derived by stripping the modifiers from the terms and will include synonyms and lexical variants of the terms. The quality of the representation will be measured by a concept match score based on hierarchical classification relationships The stuart extension of the McNemar test for correlated proportions will be used to compare the systems on concept match scores. The adequacy of the representation for clinical use will be judged by a survey of MDs (n=30) and NPs (n=30) experienced in clinical care of patients with HIV disease. The findings of the proposed study have the potential to identify areas for extension and refinement of the existing standardized coding and classification systems. Secondarily, a product of the study will be the development of a lexicon of clinical terms used by MD and NP providers in HIV ambulatory care, along with the best "matching" standardized terms from among the existing standardized coding and classification systems in a format that is exportable for use in other sites.